1. Field of the Invention
The present invention relates to an optical recording medium such as an optical disc, magneto-optical disc, information card or the like where data are recorded with pits formed in tracks on its recording surface, and also to a method for playback of such a recording medium.
2. Description of the Related Art
A pit format on an optical recording medium such as an optical disc rotated at a predetermined constant angular velocity (CAV) is so formed as shown in FIG. 1 for example. In this exemplary pit format, pits 71 each having a width of 0.5 .mu.m and a length of 0.86 .mu.m are formed on tracks arrayed at a pitch of 1.6 .mu.m in the radial direction. Such dimensions are set on the basis of restrictions in manufacture, the size of a playback beam spot 72 on the recording surface of an optical recording medium and so forth.
In this specification, the term "pit" signifies any of physically shaped pits such as an embossed one on a read-only optical disc, a bubble-like one on a write-only optical disc, or any of those represented by changes of the reflectivity on a phase-change type optical disc, or a pit optically detectable by utilizing the Kerr effect as on a magneto-optical disc.
For raising the recording density on such recording medium, it is generally customary to adopt techniques of narrowing the track pitch or shortening the pit length while reducing the spot size of a light beam so as to prevent generation of crosstalk that may be caused by the pits on adjacent tracks.
However, since the spot size of the light beam is proportional to the wavelength of the beam and is in inverse proportion to the numerical aperture (NA) of an objective lens, it becomes necessary to develop a novel laser light source of a short wavelength and to employ an expensive large-diameter lens for increasing the NA. .Practically, however, there exist some limits in realizing a higher recording density by such techniques, and difficulties are unavoidable in attaining a remarkably enhanced density.
For achieving a higher recording density, there may be contrived a means of narrowing the track pitch while maintaining the desired spot size and diminishing the pit width in a manner to prevent occurrence of any crosstalk. In such a case, however, other problems arise as a result of diminishing the pit width, including that the yield rate in manufacture is lowered and the conventional cutting apparatus cannot be used.